Low-Altitude Reflection via UAV-Mounted Rotatable IRS

TL;DR

This paper investigates a UAV-mounted rotatable IRS with directive elements for low-altitude communication enhancement, formulating an optimization for placement and orientation that significantly improves signal quality in 6G networks.

Contribution

It introduces a novel model of IRS elements with directive radiation patterns and derives closed-form solutions for optimal element orientation and phase shifts, optimizing UAV placement.

Findings

Directive IRS elements yield higher SNR gains.

Optimal boresight aligns with the internal angular bisector.

Trade-off identified between directional gain and path loss.

Abstract

Low-altitude network is a key enabler for extending coverage and recovering connectivity in 6G systems, especially when terrestrial infrastructure is unavailable. This paper studies a uncrewed aerial vehicle (UAV)-mounted rotatable intelligent reflecting surface (IRS) as a low-altitude reflector between a blocked base station (BS) and a ground terminal (GT). Unlike the conventional isotropic-element assumption, each IRS element is modeled with a hemispherical directive radiation pattern, whose boresight can be adjusted via element rotations. We formulate a new optimization problem that jointly designs IRS phase shifts, per-element rotation vectors, and UAV placement to maximize the received signal-to-noise ratio (SNR). Leveraging the problem structure, we derive closed-form solutions for phase alignment and element rotations, showing that the optimal boresight points are along the internal angular bisector between the BS-IRS and GT-IRS directions. With these closed forms, the design reduces to a placement optimization problem over a box-constrained airspace; we solve it using an efficient projected gradient algorithm with majorization-minimization update and a global Lipschitz constant. Numerical results demonstrate substantial SNR gains from directive elements and reveal a fundamental trade-off between directional gain and path loss, yielding useful insights into low-altitude deployment of UAV-mounted IRSs.